Geek time: PTU and RDU patent document - 2014+ Jeep Cherokee Forums
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post #1 of 8 (permalink) Old 04-11-2017, 04:54 PM Thread Starter
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Geek time: PTU and RDU patent document

I found this linked on some other Jeep forum (can't remember which one) and have not come across it on this one, so here you go:

http://www.freepatentsonline.com/8986148.html

It describes in wonderfully deep detail all the little bits and pieces in our drivetrains and how they work together. It doesn't call out Jeep specifically, but what other FWD-based AWD vehicle out there has geared low range on both axles?
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post #2 of 8 (permalink) Old 04-11-2017, 05:22 PM Thread Starter
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Oops, guess it was: AD1/AD2/ADL Uncovered
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post #3 of 8 (permalink) Old 04-11-2017, 05:35 PM
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Oops, guess it was: AD1/AD2/ADL Uncovered
Quite alright. I hadn't seen the original post, so it was good to see it come up again.

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post #4 of 8 (permalink) Old 04-12-2017, 03:05 PM
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Originally Posted by iwfur25 View Post
Oops, guess it was: AD1/AD2/ADL Uncovered
Agree, that is a very interesting patent document. At the time, I wasn't sure if American Axle's EcoTrac was the system used in the KL. Then later I saw 2013 FCA and American Axle press releases that confirmed.

After studying the patent, it wasn't clear how torque vectoring is accomplished using the TTD. Then I stumbled on a patent that suggests how it might be done: Torque Split Uncovered.
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post #5 of 8 (permalink) Old 04-14-2017, 04:13 PM Thread Starter
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I think of the whole system operation like this... nothing really happens torque-vectoring wise until a wheel starts to slip. With the rear end disengaged, the torque being produced equally distributed between the 2 front wheels. Engaged it's equal to all 4 wheels. Under normal driving at part throttle in dry conditions, you're never outputting enough torque to break traction on any wheel whether it's 2 or all 4 engaged. The magic doesn't happen until something starts to slip.

A key thing is that the front axle cannot be disconnected from the transmission output ever. Well, except in the PTU neutral mode but that doesn't get used during driving. There is no clutch or anything to modulate how much torque is available to the front axle. This is why the system is said to be "FWD biased/based/etc". The front axle is engaged at all times through interlocked steel bits to the transmission output. The front axle has the POTENTIAL to put 100% of the available torque to the ground at any given time if traction allows, regardless of drive mode.

When Jeep mentions 40/60 or 60/40 power distribution, that's kind of generic-speak for the layperson.

Take sport mode... the reality is that 40 front/60 rear means that 60% of torque is AVAILABLE to the rear when in that mode. Front wheels on ice, ball bearings, a giant lube spill... the power transfer clutch is pressurized enough to allow 60% of the torque to flow to the rear axle. But this doesn't happen until there is front wheel/axle slip. All driven wheels are going to be equally taking the torque until one slips. With no wheel slip, there is also no slip at the power transfer clutch. Both sides of it are turning at equal speeds and there's no torque biasing going on. Of course the clutch slips a little when you're turning but that's a relatively small amount compared to what happens when a wheel spins on slick surface.

Or say you're on a dry road and punch it from a stop. In sport mode, so the rear is engaged and ready. For the first split second, there's not enough torque to break traction. Torque distribution is equal to all 4 wheels. Very soon though, as you get higher in the power band, a front wheel is logically going to try to break loose first. It'll start to slip but with 60% available to the rear through the transfer clutch, spin is going to be instantly arrested (like you might hear a little tire scrub up front but that's it) as the transfer clutch starts to slip. Realistically it probably only takes like 20% of available torque to be put down thru the rear axle to arrest any spinning up front because let's face it, we're not making 500HP here .

Another way of saying it, the full 60% isn't going to go to the rear axle unless the front axle can only handle 40% (or less) of the torque being produced like on ice.

Snow mode is pretty much the same but flipped around so only 40% torque is available to the rear. They want you to have extra traction, but it's safer to not allow too much to be sent to the rear axle in order to prevent oversteer.

The 50/50 "split" in sand/mud or rock modes is pretty much just like a locked old-school transfer case. The power transfer clutch is fully pressurized, allowing 100% of available torque to go to either axle as ground traction dictates. That's why Jeep says "100% available to the rear axle". It's still 100% available to the front axle too though, like if you're dragging yourself up over a rock shelf and the rear is in the air.

I believe in rock mode it's allowed to slip a little, as the owners manual calls out "greater steering ability" in rock mode. That implies some allowed slip between the axles so there is less tire scrubbing on high traction rocks.

And of course on top of all this, the transfer clutch is fully variable so I'm sure the computer is able to shuffle the torque transfer ratio around in the various drive modes like auto/sport/snow depending on what it detects is going on traction and stability-wise.

Last edited by iwfur25; 04-14-2017 at 04:39 PM.
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post #6 of 8 (permalink) Old 04-14-2017, 07:09 PM
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Three years ago a forum member got a response from Jeep on torque biasing: Selec Terrain mode power distribution. The statement "amount of torque reacted by the rear axle" is interesting. Assuming the term "reacted" in that context is associated with Newton's 3rd law, this could have implications for a vehicle going through a turn where the rear wheels are cutting a wider curve (turning slightly faster) than the front wheels. Wouldn't pulsing the TTD send torque from the rear to the front? (I don't pretend to be able to visualize this.) This might be used to actively improve "spirited" driving.

While friction at the wheels is a factor, Jeep suggests there are other factors including distribution of mass. We've also heard the KL has yaw control, so the computer may be factoring yaw into the algorithm that controls the TTD.

Here's the statement allegedly from Jeep:

"The 2014 Jeep Cherokee utilizes its active 4WD clutch to influence the
amount of torque reacted by the rear axle, as the actual amount is a
function of the clutch, the friction at the tires and the center of the
vehicle's mass.
Because it is active, it has the ability to split the
torque at a variety of levels (40/60, 50/50, 60/40) and, consequently
the best control for each particular situation. This is especially
important for spirited driving
and off-road maneuvers that challenge
many SUVs in this segment; however, the Cherokee can handle them with
ease.

This can be accomplished a few ways depending on how the vehicle is
equipped:
-A center differential which can provide a fixed torque split
-An open differential would be a constant 50/50 f/r torque split
-Torque Biasing Differential helps to mitigate axle slip
-Planetary differential provides a fixed torque split other than 50/50
-Actively controlled 4WD clutch which determines the target amount of
torque to be reacted by the rear axle.


When necessary, the Power Transfer Unit and Rear Drive Module
distributes power variably between the front and rear wheels. When in
Sport Mode, the power is variably split between the front and rear
wheels with a rear wheel biased 40/60 split."

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post #7 of 8 (permalink) Old 04-15-2017, 07:40 PM Thread Starter
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Yeah, "torque reacted by the rear axle" is an interesting way of putting it. Still sounds like 60% rear means 60% available to go back there if needed, not 60% being "sent" there. If the front can't "react" whatever torque is attempting to go through it (no traction) up to 60% can be reacted by the rear.

In a corner, where the 4wd clutch is slipping a bit (different axle speeds) I can see the amount of clutch lockup affecting handling a little bit. The front axle is moving faster and tries to "drag" the rear up to speed through the clutch so it's definitely going to feel different than if it were in FWD mode.

The last bit is still kinda misleading...

"When necessary, the Power Transfer Unit and Rear Drive Module
distributes power variably between the front and rear wheels. When in
Sport Mode, the power is variably split between the front and rear
wheels with a rear wheel biased 40/60 split."

That would make perfect sense if the the RDU were being directly powered by the engine (kinda like a traditional transfer case) with 2 outputs, one to the front, one to the rear each with it's own variable clutch. But it's not. It's a "slave" if you will to the front axle. It can't control how much torque is "sent" to the front axle... it can only control how much is allowed to be applied to the rear axle in the event the front is on something slick and can't put all the torque down on the ground. It really should say "UP TO a rear wheel biased 40/60 split" with 60% rear only occurring if the front is on a surface that will only accept 40%.

But regardless of what it's doing, it works really well . I'm glad and kinda surprised Jeep went out of the way to make a crossover better than it probably needed to be.
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Last edited by iwfur25; 04-15-2017 at 08:18 PM.
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post #8 of 8 (permalink) Old 04-15-2017, 08:36 PM Thread Starter
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Small sidenote, basically all transverse engined FWD-based AWD vehicles work this way. The front axle is the "primary" and the magic AWD system allows torque to go to the rear if needed when the front slips. That device tends to be just like ours too. A simple power take off up front and a variable clutch in the rear. Jeep just seems to have nailed the fine tuning to make it behave very well off road.

This article describes transverse systems like ours very well:

http://www.thetruthaboutcars.com/201...-differential/

My favorite sentence: "Systems like this are incapable of sending more than 50% of the power to the rear unless the front wheels have zero traction." Makes perfect sense to me. As front traction drops, the rear takes up the slack. In our case, the rear can take all of it if needed.

Last edited by iwfur25; 04-15-2017 at 09:07 PM.
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